Phasmids columnar phases

Columnar phase - phase that is formed by stacking of molecules in columns. Note that sugars, etc. are not necessarily discotic dis-cotic reflects a disc-like molecular shape. Also phasmids are columnar, but not necessarily discotic. 

Symmetrically substituted linear trimers also exhibit the odd-even effect [232 a]. Twins with phasmidic-like molecular structure occur in columnar phases [232b]. 

Rod-like liquid crystals [1] have been known for more than a hundred years, the first one, cholesteryl benzoate, being discovered in 1888 by Reinitzer. In materials of this type, nematic N, cholesteric N and different lamellar mesophases such as SmA, SmC, SmF, and SmI are obtained. Beside these classical liquid crystals, thermotropic mesophases - consisting of two-dimensional aromatic flat molecules - that exhibit various columnar phases (e.g., Col, Coif, Colob) have been known since 1977 [2, 3], In these two types of systems, the lamellar and columnar phases are observed separately. So, it was interesting to examine the mesomorphic properties of the hybrid molecules, i.e., molecules with a long rodlike rigid core ending in two half-disc moieties (Fig. 1). In fact, the phasmids [4,5] fill... 

The first studies were performed on phasmids 5 (Scheme 1) with two different chain lengths the short chain derivative exhibits only a hexagonal columnar phase while that with longer chains provides... 

The main area per paraffinic chain increases with temperature and chain length, corresponding to an increase of the curvature between the aromatic and the paraffinic moieties. For this reason columnar mesomorphism is always the high temperature one and is obtained directly from the isotropic liquid. Correlatively, with two dense paraffinic ends (phasmids and pentacatenars) a columnar phase occurs, while smectic and nematic phases appear as the number of paraffinic chains is decreased [50]. 

Oblique and Rectangular Phases The first example of an oblique columnar phase has been observed on the first phasmid described (5, = 7, Table 1). A central terephthalylidene ring fitted with two ben-zoyloxyphenylimines seems to favor phases (Tables 1, 5 and 9). Regarding the rectangular columnar mesophase, chain-rich polycatenars such as phasmids do not display this mesomorphic symmetry, which so far has appeared only in the biforked tetraester series 19. 

Molecules which combine the features of the rod and the disc may be expected to form new types of mesophases. An example is the biaxial nematic phase reported in thermotropic systems (see 6.6). Malthete et a/. have prepared an interesting series of mesogens shaped like stick insects called phasmids (fig. 6.1.5(n)). Some of them form columnar mesophases the structure proposed for the hexagonal phase is shown schematically in fig. 6.1.5( >). 

In the early development of liquid crystals, for the most part, the study of small molecular systems dominated the field because of the close link between molecular design and commercial applications. However, it is only in the last 20 years that materials with unusual, and often hybrid structures have been investigated for their liquid-crystalline behavior. As noted, phasmidic materials, which have molecular structures that are part-disc part-rod, were found to exhibit both columnar and smectic phases. More recently, molecular systems having bent-rod-like structures have been investigated and found to exhibit a wide range of novel phases, many of which were found to be ferroelectric or antiferroelectric (without molecular chirality) due to the reduced symmetry of their mesophase structures. 

The few pentacatenar compounds that have been described in the literature all exhibit columnar (phasmidic) phases [21]. Typical examples of hexacatenar materials are presented in Table 40 [21], Despite their similarity of having five benzene rings in the... 

Hexagonal columnar (phasmidic) phase Oblique columnar (phasmidic) phase. 

Malthete and coworkers [6,29] have prepared a series of mesogens shaped like stick insects called phasmids (8). Some of them form columnar mesophases the structure proposed for the hexagonal phase is shown in Figure 9. 

Other liquid crystal phases composed of molecules having unusual molecular shapes are also known. Some compounds form a cone or bowl shape. These materials are often referred to as bowlic, and some typical examples are hexasubstituted tribenzocyclonones and aza-crowns (17), molecules of which stack on top of each other to form an ordered column which may have anomalous electrical conductivity. When a disklike molecule is divided by a rod-shaped spacer, the molecule (18) is referred to as a phasmid. It exhibits phases bearing some resemblance to columnar discotics. 

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